Here is some history and recommended celebrations I found at the AAAS (Amer. Assoc. for the Advancement of Science) website:

Today is National Pi Day! In case you haven’t figured out the connection yet, it’s 14 March—written 3/14 in the American calendar system—and we get to celebrate the number pi, 3.14159, by eating pie, (preferably at 1:59 p.m.). Pi Day has been unofficially celebrated by universities, institutions, and miscellaneous science geeks for years—the earliest official celebration on the books happened at the San Francisco Exploratorium in 1988—but the House of Representatives officially designated 14 March as National Pi Day in 2009. Science is commemorating today’s festival of math and dessert with a party. How are you celebrating? Tell us in the comments.